Re: better atomics - v0.5

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I'm personally not convinced that we're approaching this topic in the
> right way. I'm not convinced that it's at all reasonable to try to
> emulate atomics on platforms that don't have them. I would punt the
> problem into the next layer and force things like lwlock.c to have
> fallback implementations at that level that don't require atomics, and
> remove those fallback implementations if and when we move the
> goalposts so that all supported platforms must have working atomics
> implementations. People who write code that uses atomics are not
> likely to think about how those algorithms will actually perform when
> those atomics are merely emulated, and I suspect that means that in
> practice platforms that have only emulated atomics are going to
> regress significantly vs. the status quo today.

I think this is a valid objection, and I for one am not prepared to
say that we no longer care about platforms that don't have atomic ops
(especially not if it's not a *very small* set of atomic ops).

Also, just because a platform claims to have atomic ops doesn't mean that
those ops perform well. If there's a kernel trap involved, they don't,
at least not for our purposes. We're only going to be bothering with
installing atomic-op code in places that are contention bottlenecks
for us already, so we are not going to be happy with the results for any
atomic-op implementation that's not industrial strength. This is one
reason why I'm extremely suspicious of depending on gcc's intrinsics for
this; that will not make the issue go away, only make it beyond our
power to control.

regards, tom lane